This invention relates generally to a control for an integrated heating system and more particularly, to a modulated control for an integrated heating system for space heating and tankless domestic hot water heating which utilizes an infrared burner module and a heat exchanger coil.
In heating systems for homes and commercial buildings, central furnaces to heat a space all operate on the same general principle. Air for a space to be heated circulates through a closed system generally comprising sheet metal ductwork, and is heated either as it passes through a heat exchanger in contact with a burning fuel, or as it passes in contact with a secondary fluid which has been heated by a burning fuel. Since burning the fuel results in the production of noxious combustion gases having exhaust temperatures which can exceed 500.degree. F., it is necessary to exhaust the combustion gases through a chimney or flue to the atmosphere. These systems are relatively inefficient as evidenced by the high exhaust temperatures of the flue gases, and costly due to the construction of the necessary flue or chimney.
Indirect fired furnaces, ones in which the air being heated is not contacted directly by the combustion gases generated, are generally used in both forced air systems and hydronic systems.
A forced air system consists primarily of a heat exchanger having combustion chambers arranged in relation to the flow of air to be heated such that fuel is introduced at one end of a chamber where a flame causes heat to be generated. The heat passes through a series of internal baffles before exiting through the other end of the combustion chamber into the flue or chimney. Simultaneously, circulated space air passes around the outside of the heat exchangers to absorb heat through conduction and convection.
A hydronic system consists primarily of a firebox having a heat exchanger therein. The heat exchanger is in a closed loop for continuously circulating water, a water glycol solution or other suitable heat exchange medium from the heat exchanger to a remote radiator in the space to be heated. However, this system is also relatively inefficient and expensive due to the combustion gas temperatures at the outlet of the firebox and the cost of the chimney.
Thus, the inefficient home heating system is generally the largest consumer of energy with the domestic hot water system being the second largest consumer of energy. In supplying domestic hot water for homes and commercial buildings, potable hot water systems with ordinary glass-lined, hot water storage tanks are generally used. It is common for these systems to have an enclosed water tank in which are spiraled coils of tubing through which flows the water to be heated. At the lowermost portion of the tank there is normally a burner whose heat is allowed to pass over the coils, thereby heating the water in the tank for use within the home or building. Again, as in the space heating systems for homes and buildings, the heat which is not transferred to the heat exchanger during demand "on-time" and also during standby "off-time", is exhausted at the top of the tank into a flue or chimney to the atmosphere as well as being lost through the tank jacket. Thus, a domestic hot water system is also inefficient because a great portion of the heat is lost directly up the chimney to the atmosphere.
Because of the rising costs of energy, the incentives to conserve energy are increasing. Consequently, there is currently considerable interest in recovering energy, such as waste heat from combustion heaters which is usually injected into the atmosphere without recovery.
In an attempt to reclaim reject heat, heat exchanger coils have been installed in the flue of a furnace to transfer some of the waste heat to domestic hot water heaters, thus recovering some usually wasted heat.
However, a drawback to conserving energy by reclaiming reject heat from a furnace for use by domestic hot water heaters is that both systems are controlled independently, and the energy saved is limited by the temperature of the water in the hot water tank for potable use and typically maintained between 120.degree. F. and 160.degree. F., the average being at or above the flue gas condensing temperature therefore limiting the efficiency of recovery at or up to a maximum threshold of the product of 88% to 90%. The necessity for dual control schemes for semi-integrated furnaces and hot water heaters is due to the blue flame burners used by both systems. In semi-integrated appliances dual controls are necessary because there is not true integration of a common heating loop that provides capacity at different required temperatures for both heating and hot water. This requires a rapid on-off response with modulation of input and flow controls and blue-flame burners by nature are not capable of controlling modulation this way effectively and therefore are limited to operation at some fraction of full input during continuous operation. Capacity of these burners cannot be reduced as demand for hot water is reduced but are fired at full capacity under all operating conditions.
Thus, there is a clear need for an integral liquid-backed gas-fired heating and hot water system having a modular design and a capacity control scheme for the integrated system.